Arc-quenching fuse tubes are well-known in the art and are typically used with electrical cutouts or similar equipment to suppress and/or quench electrical arcing. Arcing can occur when fuse link melting is induced by a fault during operation of an electrical system. To restore normal operation of the system, it is desirable to suppress the arc and clear the fault. Fuse tubes may serve this purpose, and are preferably capable of suppressing and removing arcing conditions repeatedly.
Fuse tubes, and especially the inner surfaces of fuse tubes, are typically formulated from horn fiber, also referred to as bone fiber. Horn fiber is a naturally-occurring substance and is composed largely of keratinous material, which is a tough, fibrous protein. Upon exposure to an electrical arc, horn fiber can decompose, typically via ablation or vaporization. This decomposition generally results in the rapid generation and evolution of gases which interrupt and quench the electrical arc. Horn fiber also possesses desirable mechanical strength and is generally capable of withstanding the high temperature and pressure conditions that can be created by electrical arcs.
Despite the various benefits of horn fiber, including those described above, there are many undesirable drawbacks associated with horn fiber. In this connection, the supply of horn fiber is generally very limited, and its continued availability is uncertain. The manufacture of horn fiber and products which contain horn fiber, such as fuse tubes, is difficult and time-consuming. This tends to increase the cost of horn fiber and horn fiber products.
Generally, fuse tubes contain a liner formulated from horn fiber with a surrounding layer or shell of a synthetic polymeric resin and/or glass fiber. Difficulty has been encountered in achieving a satisfactory bond between the horn fiber liner and this outer layer. In most cases, only a weak mechanical bond can be achieved. Horn fiber is undesirable for this reason also.
Due to the various drawbacks associated with horn fiber, including those discussed above, attempts have been made to develop fuse tubes from materials other than horn fiber. For example, Mattuck et al., U.S. Pat. No. 4,373,555 and Bergh, U.S. Pat. No. 4,373,556, generally disclose cutout fuse tubes which comprise a core or lining of an epoxy resin reinforced with at least about 45% by weight of a polyester fiber. Aluminum trihydrate is incorporated in the Mattuck et al. fuse tubes in an amount of no more than 15% by weight. Although described as a flame retardant, aluminum trihydrate would have very limited flame suppression characteristics at the concentrations disclosed, and would contribute very little, if any, to arc extinguishment.
Fuse tubes in which higher amounts of aluminum trihydrate are incorporated in a synthetic resinous core are disclosed in Rinehart, U.S. Pat. No. 5,015,514. The Rinehart patent teaches the incorporation in the inner core of from about 40% to about 80% by weight of aluminum trihydrate. Such high amounts of aluminum trihydrate can create significant processing difficulties during manufacture of the fuse tubes including, for example, significantly increased viscosities of the resinous compositions. This high viscosity creates handling problems and mixing difficulties, and increased processing times.
Difficulty has also generally been encountered in the manufacture of fuse tubes from synthetic resins, irrespective of the presence of aluminum trihydrate. In this connection, fuse tubes manufactured from synthetic resins are typically manufactured by drawing a fiber, for example, a polyester fiber, through a resin formulation. The resin-coated fiber is then wound, for example, around a mandrel. It is generally desirable to minimize the formation of gaps between adjacent turns of the coated fiber on the mandrel inasmuch as gaps can deleteriously affect the arc-quenching properties of fuse tubes, and ultimately lead to their failure. Methods for preparing fuse tubes from synthetic resins and fibrous materials in which gaps are substantially prevented have generally been unavailable heretofore.
Accordingly, new and/or better fuse tubes and methods for their preparation are needed. The present invention is directed to these, as well as other important ends.